Miniature antenna and electromagnetic field sensing apparatus

ABSTRACT

An electromagnetic field sensing apparatus includes an optical modulator, a miniature antenna for sensing the electromagnetic field, a light source, a first optical fiber, an optical detector and a second fiber. The optical modulator can vary the amplitude of a light beam propagating therethrough in response to an electric field intensity. The miniature antenna for sensing the electromagnetic field can sense both the electric field intensity and the magnetic field intensity of an electromagnetic field and apply an electric voltage to the optical modulator in response to the sensed electromagnetic field intensity. The light source is used to generate a light beam, the first optical fiber is used for transmitting the light beam to the modulator, and the second fiber is used for transmitting the light beam from the modulator to the optical detector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a miniature antenna and anelectromagnetic field sensing apparatus, and more particularly, to aminiature antenna and an electromagnetic field sensing apparatus forsensing an electric field signal and a magnetic field signal of anelectromagnetic field.

2. Description of the Related Art

Conventional sensors for sensing an electromagnetic field (antenna forinstance) use a cable to transmit signals. However, the electromagneticfield will be interfered by the cable since the cable itself isconductor, and the so-called optical sensor for sensing theelectromagnetic field is developed to solve this problem.

The optical sensor for sensing the electromagnetic field usually usesthe Nd:YAG laser as the light source, and uses the LiNbO₃ crystal as thesubstrate on which optical waveguides are made for forming interferingsignals (See “IEEE Transactions on electromagnetic compatibility, vol.34, No. 4, 1992, pp. 391–396”). In addition, the Japanese corporationTokin has published more than 10 related patents, which primarily relateto technology for designing and manufacturing the optical modulator ofthe electric field sensing device, and technology about the thermalcompensation with the optical fibers (See “EP0664460B1, EP0668506A1 andEP0668507A1”). However, these papers and patents only focus on theelectric field sensing device, and the magnetic field sensing device isnot discussed at all.

FIG. 1 is a schematic diagram of an optical apparatus 10 for sensing anelectric field according to the prior art. As shown in FIG. 1, theoptical apparatus 10 for sensing the electric field uses an electricfield antenna 12 to sense the electric field signals of theelectromagnetic field to be sensed. The output port of the electricfield antenna 12 is connected to an optical modulator 14, whichcomprises one optical input waveguide 16, two optical phase modulationwaveguides 18 and one optical output waveguide 20. The optical modulator14 uses the LiNbO₃ crystal, and there are two electrodes 24 and 26positioned above the optical phase modulation waveguide 18.

The semiconductor laser emitted from the light source is conducted intothe optical input waveguide 16 via the first optical fiber 22, entersthe optical phase modulation waveguide 18 after light splitting, and isthen ultimately merged to the optical output waveguide 20. The voltagedifference between the electrode 24 and electrode 26 shall change therefractive index of the optical phase modulation waveguide 18, and thephase of the laser propagating through the two optical phase modulationwaveguide 18 is changed, i.e., the phase difference is changed. As aresult, the output amplitude of the laser from the optical outputwaveguide 20 changes with the voltage difference between the electrode24 and the electrode 26. When the electric field antenna 12 receiveselectric field signals, its output electric field signals shall modulatethe amplitude of the output laser from the optical output waveguide 20.Therefore, electric field signals are converted into light signals onthe optical modulator 14, and the optical signal is then transmitted tothe optical detector 30 via the second optical fiber 28. Subsequently,the interference problem is solved since there is no cable.

However, a magnetic field sensing device is needed to sense the magneticfield since the electric field signals measured at the near field failto represent the whole electromagnetic field. In addition, the opticalsensor for sensing the electromagnetic field generates the zero drift asthe environmental temperature varies. Although the zero drift can beovercome by connecting a temperature sensing device with a conductivewire and controlling the electric field sensing device according to thetemperature variation feedback from the temperature sensing device, theconductive wire shall also influence the electromagnetic field to besensed. Therefore, it is necessary to develop a compensation technologyfree of conductors.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a miniature antennaand an electromagnetic field sensing apparatus for sensing an electricfield signal and magnetic field signal of an electromagnetic field.

In order to achieve the above-mentioned objective, and avoid theproblems of the prior art, the present invention provides a miniatureantenna and an electromagnetic field sensing apparatus. Theelectromagnetic field sensing apparatus comprises an optical modulatorfor changing the amplitude of a light beam propagating therethrough inresponse to an electric field intensity, a miniature antenna for sensingthe intensity of the electromagnetic field and applying an electricfield to the optical modulator in response to the intensity of theelectromagnetic field, a light source for generating the light beam, afirst optical fiber for transmitting the light beam to the opticalmodulator, a second optical fiber connected to the optical modulator andan optical detector for transforming the light beam transmitted from thesecond optical fiber into an electrical signal.

The miniature antenna for sensing the intensity of the electromagneticfield comprises a first line, a first optical switch positioned at oneend of the first line, a second optical switch positioned at the otherend of the first line, a second line connected to the first opticalswitch and a third line connected to the second optical switch. Theminiature antenna forms a loop antenna for sensing a magnetic field whenthe first optical switch and the second optical switch are turned on,and forms a line antenna for sensing an electric field when the firstoptical switch and the second optical switch are turned off.

Compared with the prior art, the electromagnetic field sensing apparatusof the present invention can sense both the magnetic field signal andthe electric field signal of the electromagnetic field to be sensedusing a single miniature antenna, which is controlled by the opticalswitches.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objectives and advantages of the present invention will becomeapparent upon reading the following description and upon reference tothe accompanying drawings in which:

FIG. 1 is a schematic diagram of a prior art optical apparatus forsensing an electric field;

FIG. 2 is a schematic diagram of an electromagnetic field sensingapparatus according to the present invention;

FIG. 3 is a cross-sectional diagram of the optical modulator along A—Aline according to the present invention; and

FIG. 4 is a schematic diagram of a miniature antenna for sensing anelectromagnetic field according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a schematic diagram of an electromagnetic field sensingapparatus 40 according to the present invention. As shown in FIG. 2, theelectromagnetic field sensing apparatus 40 comprises an opticalmodulator 50, a miniature antenna 70 for sensing the intensity of theelectromagnetic field, a light source 80 of semiconductor laser, a firstoptical fiber 82, an optical detector 84 and a second optical fiber 86.The miniature antenna 70 senses the intensity of the electromagneticfield signals and applies an electric field on the optical modulator 50in response to the sensed intensity of the electromagnetic field, whilethe optical modulator 50 changes the amplitude of the light beampropagating therethrough in response to the electric field applied bythe miniature antenna 70. The light source 80 is used to generate alight beam such as a laser, and the first optical fiber 82 is used totransmit the light beam generated by the light source 80 to the opticalmodulator 50. The optical detector 84 is used to convert an opticalinput signal into an electric signal, and the second optical fiber 86 isused to transmit the light beam output from the optical modulator 50 tothe optical detector 84. In addition, the optical modulator 50 comprisesan optical input waveguide 51, two optical phase modulation waveguides52 and an optical output waveguide 53.

The miniature antenna 70 comprises a first line 71, a first opticalswitch 74 positioned on one end of the first line 71, a second opticalswitch 75 positioned on the other end of the first line 71, a secondline 72 connected to the first line 71 though the first optical switch74, and a third line 73 connected to the first line 71 through thesecond optical switch 75. When the first optical switch 74 and thesecond optical switch 75 are turned on, the first line 71, the secondline 72 and the third line 73 form a loop antenna for sensing magneticfield signals, while when the first optical switch 74 and the secondoptical switch 75 are turned off, the second line 72 and the third line73 form a line antenna for sensing electric field signals. The opticalfibers 76 and 77 are used to transmit switching signals to control theon/off of the first optical switch 74 and the second optical switch 75.

FIG. 3 is a cross-sectional diagram of the optical modulator 50 alongA—A line according to the present invention. As shown in FIG. 3, theelectrodes 54 and 55 are positioned above the optical phase modulationwaveguide 52, and there is an electrode 56 between electrodes 54 and 55.The electrodes 54 and 55 are electrically connected to one output portof the miniature antenna 70, while the electrode 56 is electricallyconnected to another output port. When the miniature antenna 70 sensesthe intensity of the electromagnetic field to be sensed, it converts theintensity signal into an electric field signal, and applies an electricfield (voltage difference) to the electrodes 54, 55 and 56 in responseto the electric field signal.

The light beam emitted from the light source 80 is conducted into theoptical input waveguide 51 via the first optical fiber 82, enters theoptical phase modulation waveguide 52 after light splitting, and then isultimately merged to the optical output waveguide 53. The voltagedifference between the electrodes 54, 55 and electrode 56 shall changethe refractive index of the optical phase modulation waveguide 52, andthus change the phase difference of the light beams propagating throughthe two optical phase modulation waveguides 52. Therefore, the amplitudeof the output light from the optical output waveguide 53 changes withthe voltage difference between the electrodes 54, 55 and 56.

The above structure details of the optical modulator 50 are based onX-cut LiNbO₃ optical modulator. If the structure of the opticalmodulator 50 changes, for example, it changes into Z-cut LiNbO₃ opticalmodulator, these details will be changed accordingly.

FIG. 4 is a schematic diagram of the miniature antenna 70 according tothe present invention. As shown in FIG. 4, the miniature antenna 70 isformed on a substrate 90, such as a substrate made of mica, and it canbe a circular loop or a rectangular loop. In addition, three groups ofminiature antenna 70 can be positioned on three orthogonal surfaces for3-dimensionally polarized electromagnetic field measurement.

After the miniature antenna 70 has sensed the electromagnetic field, theelectromagnetic field signal is loaded onto the light beam through theoptical modulator. Temperature shift will generate a drifting of thephase difference between the two optical phase modulation waveguides 52,and results in variation of the modulation sensitivity and linearity ofthe electromagnetic field signal. Subsequently, the intensity of thesignal is changed and error occurs. When the temperature variationresults in phase difference of the light beam, which further shifts thelevel of the interfering signal, the present invention can measure theaverage level of the output optical signals, acquire the DC component ofthe electric signals converted by the optical detector 84, and feedbackto the light source 80 by the laser wavelength controller 88 to controlthe wavelength. As a result, the level of the interfering signal can bemaintained in a predetermined level to eliminate the zero drift.

Compared with the prior art, the electromagnetic field sensing apparatus40 of the present invention can sense both the magnetic field signal andthe electric field signal of the electromagnetic field to be sensedusing a single miniature antenna 70, which is controlled by the opticalswitches 74, 75.

The above-described embodiments of the present invention are intended tobe illustrative only. Numerous alternative embodiments may be devised bythose skilled in the art without departing from the scope of thefollowing claims.

1. A miniature antenna, comprising: a first line; a first optical switchpositioned at one end of the first line; a second optical switchpositioned at the other end of the first line; a second line connectedto the first optical switch; and a third line connected to the secondoptical switch; wherein the first optical switch and the second opticalswitch are turned on for sensing a magnetic field and turned off forsensing an electric field.
 2. The miniature antenna of claim 1, beingformed on a mica substrate.
 3. The miniature antenna of claim 1, whereinswitching signals are transmitted between the first optical switch andthe second optical switch by an optical fiber.
 4. The miniature antennaof claim 1, wherein the second line and the third line are formedsubstantially in straight.
 5. The miniature antenna of claim 1, whereinthe first line, the second line and the third line are formedsubstantially as a loop.
 6. The miniature antenna of claim 5, whereinthe loop is rectangular or circular.